Problem: $ { {3} \times \left[\begin{array}{r}{1} \\ {2} \\ {4}\end{array}\right] = {?} } $
Explanation: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{3}\times{1} \\ {3}\times{2} \\ {3}\times{4}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{3} \\ {6} \\ {12}\end{array}\right]}$